High capacity electric hydropower generators are often liquid-cooled. For this purpose, the windings of the stator and/or rotor, and under circumstances the lamination stack, too, are provided directly or indirectly with cooling channels through which the coolant, usually water, is conducted. The pump-driven coolant, which must be specially prepared so it will contain no contaminants or be too conductive, circulates in a closed cooling system containing, among others, mechanical and chemical filters and conductivity control means. Also provided are means to check for possibly occurring leaks to prevent the loss of prepared coolant. To recool the coolant which was heated up during its circulation by the running hydropower generator, liquid-cooled heat exchangers are provided which are likewise included in the circulatory cooling system.
It is common practice to interrupt the coolant circulation in the cooling system as well as the supply of liquid to the heat exchangers when the hydropower generator is not in operation. Simultaneously with restarting the hydropower generator, the water cooling system is switched on again so that the cold coolant is conducted through the machine which is not yet warmed up, the consequence of which can be that the voltage-carrying components are cooled so much that condensate forms thereon; this, however, gives rise to the formation of creepage paths.